Low noise bipolar transistor

Abstract: A low-noise NPN transistor comprising a cut-off region (38) laterally surrounding, at a given distance, the emitter region (36) in the surface portion of the transistor and of such conductivity as to practically turn off the surface portion of the transistor, so that the transistor operates mainly in the bulk portion. The cut-off region is formed by a P ring (38) astride a P⁻ type well region (35) and the epitaxial layer (32).

Description: The present invention relates to a low-noise bipolar transistor. As is known, in electronic devices, the term "noise" indicates a random fluctuation in currents or voltages at the device terminals, and may seriously limit the minimum signal level that can be handled by the device. The noise in each device is due to various physical causes, some of which have been known for some time. Of particular interest are what are known as "flicker" noise (also indicated 1/f) and "burst" noise, the first of which exists in all and the second in a significant percentage of devices. Flicker noise is commonly acknowledged to be caused by fluctuations in the number of carriers, due to entrapment of the carriers in surface layers of the device, i.e. to tunneling at the semiconductor-oxide interface. According to accepted theory, the carriers in the semiconductor may communicate with trap levels at a given distance within the tunnel oxide layer, and remain trapped for some time prior to being re-emitted. In the case of transistors, in particular, flicker noise sources are located at the base-emitter junction. Flicker noise is especially undesirable in the case of operational amplifier input transistors and audio preamplifiers. Burst noise, on the other hand, is caused by a sharp variation in current between two or more constant values. Variation frequency may be very low (less than 1 Hz) or high (hundreds of Hz), in which case, burst noise may be confused with a high degree of flicker noise. This type of noise is generally attributed to the presence of defects, metal inclusions and precipitates in the space charge region of the junction; and the fluctuation in current depends on the extent, if any, to which the defect participates in conduction. The fact that burst noise is reduced by deficiency-reducing processes, such as gettering, would appear to bear out this theory. In the case of flicker noise, the noise power at the output terminals of a transistor is proportional to I B α , where I B is the base current and α a constant ranging between 1 and 2. In the case of burst noise, the output noise power is inversely proportional to the square of the gain of the transistor, so that, for a given collector current, high-gain transistors are less affected by flicker and burst noise as compared with low-gain types. One proposal already made for reducing flicker and burst noise is to produce extremely high-gain (super-beta SBT) transistors with a gain typically ranging between 1000 and 10,000 for collector currents below 1 µ A. In NPN type super-beta transistors, the base layer is narrower as compared with standard transistors, for improving the base transfer factor (reducing recombination of the charge carriers) and so increasing gain. In a planar process, in particular, by reducing the base width (thickness of the P type base layer between the N + type emitter region and the N type epitaxial layer) to roughly 0.2-0.3 µ m (as compared with the normal 0.8 µ m), current gain increases to as much as 2000-5000 (as compared with 200-300). High-gain NPN transistors may be produced simultaneously with conventional NPN types by adding a photolithography and diffusion step to the planar process. In practice, following base diffusion of the conventional NPN transistor, windows are opened photolithographically, through which the emitter of the high-β F transistors is predeposited and partially diffused. This is followed by photolithography and diffusion of the conventional emitter, so that the emitter of high-β F transistors is deeper than that of conventional ones, thus considerably reducing the thickness of the active base (by way of comparison, refer to Figures 1 and 2 relative to a standard and a high-gain transistor, in which w B and w B , indicate the respective base widths). High-gain transistors of the aforementioned type, however, present an extremely low (roughly 3 V) open-base collector-emitter breakdown voltage (BV CEO ), and cannot be employed in applications requiring a higher voltage, due to the risk of punch-through between the emitter and collector. Moreover, prediffusion increases the likelihood of emitter pipes being formed, due to penetration of an emitter portion inside the base region as far as the collector, as a result of crystallographic defects. As is known, such pipes so modify the I C -V CE output characteristics of the transistor that they become resistive. This problem is particularly felt in the case of power integrated circuits, even to the extent of eliminating the advantages of high gain, or resulting in production rejects and, hence, reduced efficiency. A further drawback lies in the need for providing an additional masking step as compared with the standard fabrication process, thus increasing fabrication costs. Another known noise reducing solution consists in producing transistors with a high A E /P E ratio, i.e. a high ratio of the area and perimeter of the emitter region visible from above, so as to reduce the contribution of the surface portions with respect to that of the deep regions (bulk). This solution, however, only provides for reducing flicker and is ineffective as regards burst noise. A high-gain, high-voltage transistor is disclosed in US-A-4,047,217, wherein, to prevent an unwanted inversion layer from appearing along the surface, an annular base region with a higher doping level than the base well surrounds and is slightly superimposed to the emitter region. Furthermore, the article "An Advanced Bipolar Transistor with Self-Aligned Ion-Implanted Base and W/Poly Emitter" published in iEEE Transactions on Electron Devices, vol. 35, no. 8, August 1988, pages 1322-1327 describes a transistor having an emitter region and an annular region surrounding the emitter region. EP-A-0 316 951 discloses a bipolar transistor having a low impurity-concentration base region. EP-A-0 553 589 discloses a NPN transistor including a P - substrate; a N + buried region provided therein; a N - epitaxial layer deposited onto the N + buried region; a P base diffusion region in the N - epitaxial layer; a N + reach-through region through the N - epitaxial layer to the N + buried region to thereby define a collector; a N ++ implant or diffusion region provided in the P base diffusion region to thereby define an emitter; and a P ++ implant region provided around the N ++ emitter implant region which thereby defines the extrinsic base of the transistor, wherein P ++ implant region extends through the P base diffusion region into the N - epitaxial layer, and wherein the P ++ implant region extends as close to the N ++ emitter implant region as possible without encroaching on the emitter. It is an object of the present invention to provide a transistor designed to overcome the drawbacks posed by known solutions, and which, in particular, provides for reducing the noise of transistors by means of a flexible solution, adaptable to various requirements, and ensuring high breakdown voltages. According to the present invention, there is provided a low-noise bipolar transistor as claimed in Claim 1. Specific embodiments are disclosed in the dependent claims. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a cross section in perspective of a silicon wafer forming a known NPN transistor; Figure 2 shows a cross section in perspective of a silicon wafer forming a high-gain NPN transistor; Figure 3 shows a cross section of a silicon wafer forming a first embodiment of an NPN transistor in accordance with the present invention; Figure 4 shows a view in perspective of the Figure 3 transistor; Figure 5 shows a cross section of a silicon wafer forming a second embodiment of the present invention; Figures 6a, 6b and 6c respectively show a third and fourth embodiment and the respective doping atom concentration profile; Figure 7a shows a cross section of a silicon wafer prior to implementing the transistor according to the present invention; Figures 7b and 7c show two concentration profiles of different portions of the Figure 7a section; Figure 8 shows a table of test results obtained by the Applicant. Figure 1 shows a standard NPN transistor with a P type substrate 1; an N type epitaxial layer 2; an N + type buried layer 3; an N + type enriched collector region 4; a P type base region 5; and an N + type emitter region 6. w B indicates the width of base region 5. Figure 2 shows a known high-gain NPN transistor with a substrate 11; an epitaxial layer 12; a buried layer 13; an enriched collector region 14; a base region 15; and an emitter region 16. As can be seen, emitter region 16 is deeper than the corresponding region 6 in Figure 1, so that base width w B , of base region 15 is less than the corresponding base width w B in Figure 1. Figure 3 shows an NPN transistor in accordance with the teachings of the present invention. More specifically, the transistor is formed in a silicon wafer comprising a P type substrate 31; an N type epitaxial layer 32; and an N + type buried layer 33 interposed between substrate 31 and epitaxial layer 32 at the active area of the transistor. Inside epitaxial layer 32, there are formed a P type well region 35 less heavily doped as compared with the others and therefore indicated P - ; an N + type emitter region 36 formed inside well region 35 and facing surface 37 of the wafer; a P type annular region 38 formed astride well region 35 and epitaxial layer 32, facing surface 37, and laterally surrounding, at a distance, emitter region-36; and an N + type enriched region 39 at the collector contact. Figure 3 also shows schematically the emitter, base and collector contacts E, B and C, and a protective oxide layer 40. To prevent stressing the semiconductor material, the Figure 3 transistor comprises no deep collector regions (normally N + type and provided between contact 39 and buried layer 33). In the embodiment shown, the distance X between emitter region 36 and annular region 38 is so selected that the width of the depletion region formed in well region 35, close to surface 37, equals the in-bulk depth of the depletion region, as shown in Figure 4 wherein the hatched portion indicates depletion region 41, and the in-bulk depth of the depletion region is indicated w D . As can be seen, in the example shown, distance X equals the width of the depletion region close to surface 37. In the Figure 3 transistor, annular region 38 (which, together with well region 35, forms the base region) presents substantially the same doping level as base region 5 of standard transistors (R S of about a hundred Ω/□)so that the peripheral portion of the base region presents the same characteristics as the standard base region 5. Conversely, well region 35 presents a much lower doping level as compared with annular region 38 (R S of a few KΩ/□), and is much deeper than the standard base region 5 (typical depths from- surface 37 for emitter region 36, annular region 38 and well region 35 are 1.5 µ m, 1.8 µ m and 3 µ m respectively). As a result, the central portion of the emitter region sees a less heavily doped and deeper portion as compared with the structure of known transistors. Well region 35 is formed prior to the base diffusion step (in this case, resulting in annular region 38) which is performed by predeposition and subsequent diffusion of boron atoms. By virtue of annular region 38 about emitter region 36, and the less heavily doped portion 35 facing emitter region 36, operation of the peripheral surface region of the transistor is reduced, and the transistor operates mainly in the bulk portion, which is deeper as compared with the known structure. As a result, in the effectively operative base region, recombination is reduced and gain increased (to 1000-1500), thus reducing both flicker and burst noise. The spacing of annular region 38 and emitter region 36 provides for minimizing base current I B and ensuring a high gain β max over the entire base-emitter junction, thus minimizing both equivalent input noise current (i in proportional to the square root of the base current) and equivalent noise voltage (e n ). The Figure 3 structure may be used in any precision circuit requiring high gain (e.g. current mirror and band-gap circuits), but in which given breakdown voltage BV CEO values must be ensured. In general, the Figure 3 structure requires an additional mask for forming well region 35. In most power circuits, however, a P-well layer is already provided at some point, so that such a mask is already included in most fabrication processes. If such is the case, the Figure 3 structure simply involves modifying the existing mask, with no need for additional fabrication steps, and hence no increase in cost, as compared with traditional processes. Even if the rest of the circuit integrated in the same chip presents no P-well regions, however, the disadvantage of providing an additional mask is more than compensated for by the considerable improvement in performance obtainable. As shown in Figure 5, epitaxial layer 32 is divided into a number of wells by P + type deep insulating regions 47 which separate region 48 integrating the bipolar transistor according to the invention from region 49 integrating resistor 45; and buried layers 33, 33' are provided respectively between substrate 31 and regions 48 and 49 of the epitaxial layer. In addition to regions 35, 36 and 38 identical to those in Figure 3, the bipolar transistor also comprises an implanted P type layer 50 completely surrounding emitter region 36 at the surface and therefore annular in shape (viewed from above), and which preferably presents a doping level midway between that of well region 35 and annular region 38 (typically of about 1 KΩ/□). Resistor 45 is formed by an implanted P type layer 51 extending between contact regions 52 connected to respective contacts K; and, being formed simultaneously using known resistor fabrication techniques, implanted layers 50 and 51 present the same characteristics. Figures 6a and 6b show two variations of the Figure 3 structure, characterized by a high degree of flexibility, high breakdown voltage, high gain, a low noise level and high cut-off frequency, due to the presence of an epitaxial layer with a variable doping profile. The only difference between the two solutions lies in the depth of the well region. More specifically, the Figure 6a and 6b structures present an epitaxial layer in two portions: a bottom portion 55 over substrate 31; and a top portion 56 extending as far as surface 37 of the wafer. Bottom portion 55 presents a doping level N 1 , and top portion 56 a doping level N 2 , where N 1 > N 2 , as shown in Figure 6c; and regions 35, 35', 38 and 39 are all embedded in the less heavily doped top portion 56 of the epitaxial layer. In the Figure 6a structure, the less heavily doped top portion 56 of the epitaxial layer reduces depletion of well region 35, so that the base-collector junction provides for better voltage holding (higher punch-through voltage between the emitter and collector) as shown in the Figure 8 table which shows the test results obtained by the Applicant with an epitaxial layer 32 of a total thickness of 11 µ m. In the Figure 8 table, R s1 and w 1 respectively indicate the resistivity and thickness of bottom portion 55 of the epitaxial layer (N 1 doping level); R s2 and w 2 the resistivity and thickness of top portion 56 (N 2 doping level); and BV PT the punch-through voltage. The top row in the table corresponds to the Figure 3 embodiment wherein R s1 and R s2 are the same. In the Figure 6b structure, appropriate selection of the doping level of top portion 56 of the epitaxial layer provides for obtaining the same improvement in punch-through voltage alongside a simultaneous improvement in one of the performance features of the transistor. A shallower well region 35' as compared with the Figure 6a structure, in fact, provides for improving performance in terms of gain, cut-off frequency and the noise level of the transistor. The Figure 6a and 6b structures can be formed quite easily, with no additional cost, using currently marketed single-wafer epitaxial reactors which, as is known, provide for obtaining a single epitaxial layer with a variable concentration profile for achieving the required precision. Any problems posed by surface inversion of top portion 56 of epitaxial layer 32 in the Figure 6a, 6b structures may be solved using a threshold correcting mask normally provided in current bipolar and BCD (Bipolar-CMOS-DMOS) processes, and which allows phosphorous ions to be implanted in the active areas requiring threshold correction prior to forming the components of the integrated circuit. In the event other integrated circuit components require an epitaxial layer with a constant doping profile (roughly equal to N 1 ), the Figure 6a, 6b solution provides for modifying the doping level of the top part of the epitaxial layer of such components by implanting an N type well as shown in Figure 7a. More specifically, Figure 7a shows a cross section of a silicon wafer prior to forming the regions forming the various components of the integrated circuit. The right-hand portion of the Figure 7a section is for integrating a low-noise NPN bipolar transistor as shown in Figure 6a or 6b, and therefore presents an epitaxial layer divided into a bottom portion 55 with an N 1 doping level, and a top portion 56 with an N 2 doping 'level lower than N 1 , as shown in the Figure 7c diagram. Conversely, the left-hand portion of Figure 7a is for housing an electric component requiring an epitaxial layer with a substantially uniform doping profile, and therefore comprises a bottom portion 55' with an N 1 doping level, and a top portion 58 with an N 3 ≈ N 1 doping level. As shown in Figure 7b relative to the doping profile of the epitaxial layer in the left-hand portion of Figure 7a, the doping level equals the sum of the original N 2 profile and the N W profile resulting from implantation of the N-well. As such, the total profile (N 1 and N 3 together) is roughly constant. Clearly, changes may be made to the transistor as described and illustrated herein without, however, departing from the scope of the present invention as defined in the appended claims.

Claims: A low-noise bipolar transistor comprising an emitter region (36) of a N type conductivity, a base region (35; 35') of a P type conductivity and a first doping level of P - and a collector region (32, 39) of N type conductivity, an annular region (38) of P type conductivity and a second doping level, higher than said first doping level, laterally surrounding said emitter region (36), said emitter region being disposed on said base region, said base region being disposed on said collector region, and said annular region adjoining said base region, said emitter, base and collector regions being integrated in a layer of semiconductor material (32) defining a surface portion and a deeper bulk portion, said annular region being located in said surface portion and laterally surrounding at least part of said base region in said surface, portion, said annular region extending at a lateral distance (x) from said emitter region (36). A transistor according to claim 1, characterized in that said base region comprises a base well portion (35, 35') embedding said emitter region (36), a portion of said base well portion being located between said annular region and said emitter region(36). A transistor according to claim 2, characterized in that said annular region (38) is deeper than said emitter region (36) and roughly half the depth of said base well portion (35). A transistor according to claim 2 or 3, characterized in that said lateral distance (X) is at least equal to the bulk depletion depth (W D ) in said base well portion (35) about said emitter region (36). A transistor according to claim 2 or 3, characterized in that said lateral distance (X) is greater than the bulk depletion depth (W D ) in said base well portion (35) about said emitter region (36) and in that it comprises a doped region (50) extending between said emitter region and said annular region (38) adjacent to the surface of said layer of semiconductor material (32), said doped region being of P type conductivity, and presenting a third doping level between said first and said second doping level. A transistor according to any one of claims 2-5, characterized in that said layer of semiconductor material (32) comprises an epitaxial layer with a variable doping profile. A transistor according to claim 6, characterized in that said epitaxial layer (32) comprises a bottom portion (55) and a top portion (56), said top portion defining the surface (37) of said transistor and housing said emitter region (36), said annular region (38) and said base well portion (35), and said bottom portion (55) presenting a higher doping level than said top portion. A transistor according to claim 2, characterized in that said base well portion (35') is shallower than said annular region (38).

Inventors: VILLA FLAVIO, VILLA, FLAVIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L29
IPC codes: H01L29, H01L21